Exhaust purification system of internal combustion engine

ABSTRACT

An exhaust purification system of an internal combustion engine has a selective reduction type NO X  catalyst device which can hold ammonia and a storage reduction type NO X  catalyst device which is arranged at an upstream side of the selective reduction type NO X  catalyst device, which makes an air-fuel ratio of exhaust gas which flows into the storage reduction type NO X  catalyst device change from a lean air-fuel ratio to a rich air-fuel ratio for starting an ammonia generation period, and which makes an air-fuel ratio of exhaust gas which flows into the storage reduction type NO X  catalyst device change from a rich air-fuel ratio to a lean air-fuel ratio for ending the ammonia generation period.

TECHNICAL FIELD

The present invention relates to an exhaust purification system of an internal combustion engine.

BACKGROUND ART

In the exhaust system of a diesel engine or other such internal combustion engine which carries out lean combustion, a NO_(X) catalyst device is arranged for purifying the NO_(X) in the exhaust gas. As such an NO_(X) catalyst device, a selective reduction type NO_(X) catalyst device is known which uses ammonia NH₃ with its high reducing ability as reducing agent so to selectively purify the NO_(X) in the exhaust gas by reduction.

The ammonia which is used as the reducing agent generally is produced by hydrolysis of urea which is supplied to the catalyst device. To eliminate the need for the supply of urea, it has been proposed to arrange a three-way catalyst device at the upstream side of the selective reduction type NO_(X) catalyst device and change the air-fuel ratio of the exhaust gas from the lean air-fuel ratio at the time of lean combustion to a rich air-fuel ratio to thereby make the three-way catalyst device sufficiently reduce the NO_(X) which is contained in the exhaust gas and produce ammonia (refer to Japanese Unexamined Patent Publication No. 09-133032).

The ammonia which is produced in the three-way catalyst device in this way flows together with the exhaust gas of the rich air-fuel ratio which does not contain almost any NO_(X) into the selective reduction type NO_(X) catalyst device which is positioned at the downstream side of the three-way catalyst device and is held at the selective reduction type NO_(X) catalyst device since the concentration of ammonia in the exhaust gas is high. Next, if the air-fuel ratio of the exhaust gas is made the lean air-fuel ratio at the time of lean combustion, exhaust gas of the lean air-fuel ratio which includes a relatively large amount of NO_(X) flows into the three-way catalyst device. At this time, the NO_(X) flows into the selective reduction type NO_(X) catalyst device which is positioned at the downstream side of the three-way catalyst device without being reduced much at all. In this way, when the ammonia concentration is low, the selective reduction type NO_(X) catalyst device releases the held ammonia and purifies the NO_(X) by reduction.

However, in the three-way catalyst device, the NO_(X) in the exhaust gas which is contained in the exhaust gas of the rich air-fuel ratio is sometimes also reduced to N₂ or only partially reduced to ammonia resulting in insufficient ammonia being supplied to the selective reduction type NO_(X) catalyst device. Due to this, arranging a storage reduction type NO_(X) catalyst device at the upstream side of the selective reduction type NO_(X) catalyst device may be considered instead of a three-way catalyst device.

A storage reduction type NO_(X) catalyst device holds the NO_(X) in the exhaust gas well when the exhaust gas is the lean air-fuel ratio. If making the air-fuel ratio of the exhaust gas the stoichiometric air-fuel ratio or the rich air-fuel ratio, the held NO_(X) is disassociated and the thus disassociated NO_(X) is reduced. Due to this, if the exhaust gas is made the rich air-fuel ratio, the storage reduction type NO_(X) catalyst device will release the held NO_(X) when the exhaust gas is a lean air-fuel ratio. If able to reduce to ammonia a part of the released NO_(X) in addition to a part of the NO_(X) which is contained in the exhaust gas, a sufficient amount of ammonia can be supplied to the selective reduction type NO_(X) catalyst device.

DISCLOSURE OF THE INVENTION

As explained earlier, in an exhaust purification system of an internal combustion engine in which a storage reduction type NO_(X) catalyst device is arranged at an upstream side of a selective reduction type NO_(X) catalyst device, if making an air-fuel ratio of the exhaust gas a rich air-fuel ratio so as to start an ammonia generation period, it is possible to ensure the presence of a relatively large amount of NO_(X) inside the storage reduction type NO_(X) catalyst device. If the storage reduction type NO_(X) catalyst device has a sufficient reducing ability, it is possible to produce a sufficient amount of ammonia from the relatively large amount of NO_(X). However, if the storage reduction type NO_(X) catalyst device is low in temperature and does not have a sufficient reducing ability, the relatively large amount of NO_(X) is reduced, due to insufficient reduction, not to N₂ and ammonia, but mainly dinitrogen monoxide N₂O and N₂. Not only cannot a sufficient amount of ammonia be supplied to the selective reduction type NO_(X) catalyst device, but also a relatively large amount of N₂O for which release into the atmosphere is undesirable ends up being produced.

Therefore, an object of the present invention is to provide an exhaust purification system of an internal combustion engine which is provided with a selective reduction type NO_(X) catalyst device which can hold ammonia and a storage reduction type NO_(X) catalyst device which is arranged at an upstream side of the selective reduction type NO_(X) catalyst device, which makes an air-fuel ratio of exhaust gas which flows into the storage reduction type NO_(X) catalyst device change from a lean air-fuel ratio to a rich air-fuel ratio for starting an ammonia generation period, and which makes an air-fuel ratio of exhaust gas which flows into the storage reduction type NO_(X) catalyst device change from a rich air-fuel ratio to a lean air-fuel ratio for ending the ammonia generation period, wherein even if the ammonia generation period is started when the storage reduction type NO_(X) catalyst device is a low temperature, ammonia is easily produced and N₂O is difficult to produce.

An exhaust purification system of an internal combustion engine as set forth in claim 1 according to the present invention is provided, characterized in that the system is provided with a selective reduction type NO_(X) catalyst device which can hold ammonia and a storage reduction type NO_(X) catalyst device which is arranged at an upstream side of the selective reduction type NO_(X) catalyst device, which makes an air-fuel ratio of exhaust gas which flows into the storage reduction type NO_(X) catalyst device change from a lean air-fuel ratio to a rich air-fuel ratio for starting an ammonia generation period, and which makes an air-fuel ratio of exhaust gas which flows into the storage reduction type NO_(X) catalyst device change from a rich air-fuel ratio to a lean air-fuel ratio for ending the ammonia generation period, wherein an amount of NO_(X) which is held at the storage reduction type NO_(X) catalyst device for starting the ammonia generation period when the temperature of the storage reduction type NO_(X) catalyst device is less than a set temperature is smaller than that when a temperature of the storage reduction type NO_(X) catalyst device is the set temperature or more.

An exhaust purification system of an internal combustion engine as set forth in claim 2 according to the present invention is provided as the exhaust purification system of an internal combustion engine as set forth in claim 1 characterized in that an interval from when ending the ammonia generation period to when next starting the ammonia generation period when the temperature of the storage reduction type NO_(X) catalyst device is less than the set temperature is made shorter than that when the temperature of the storage reduction type NO_(X) catalyst device is the set temperature or more such that the amount of NO_(X) which is held at the storage reduction type NO_(X) catalyst device for starting the ammonia generation period when the temperature of the storage reduction type NO_(X) catalyst device is less than the set temperature is smaller than that when the temperature of the storage reduction type NO_(X) catalyst device is the set temperature or more.

An exhaust purification system of an internal combustion engine as set forth in claim 3 according to the present invention is provided as the exhaust purification system of an internal combustion engine as set forth in claim 1 characterized in that when the temperature of the storage reduction type NO_(X) catalyst device is another set temperature higher than the set temperature or more, instead of the ammonia generation period, making the air-fuel ratio of the exhaust gas which flows into the selective reduction type NO_(X) catalyst device change from the lean air-fuel ratio to the stoichiometric air-fuel ratio or another rich air-fuel ratio closer to the stoichiometric air-fuel ratio than the above rich air-fuel ratio and making the storage reduction type NO_(X) catalyst device purify the NO_(X) which is disassociated from the storage reduction type NO_(X) catalyst device and the NO_(X) in the exhaust gas by reduction.

According to the exhaust purification system of an internal combustion engine as described in claim 1 according to the present invention, an amount of NO_(X) which is held at the storage reduction type NO_(X) catalyst device for starting the ammonia generation period when the temperature of the storage reduction type NO_(X) catalyst device is less than the set temperature is smaller than that when the temperature of the storage reduction type NO_(X) catalyst device is the set temperature or more. Due to this, when the temperature of the storage reduction type NO_(X) catalyst device is less than the set temperature, even if the ammonia generation period is started, only a small amount of NO_(X) is released from the storage reduction type NO_(X) catalyst device. Since there is not that large of an amount of NO_(X) present inside of the storage reduction type NO_(X) catalyst device, even if the reducing ability is low, the NO_(X) can be sufficiently reduced to N₂ and ammonia. In this way, even if the ammonia generation period is started when the storage reduction type NO_(X) catalyst device is a low temperature, it is possible to make ammonia easier to produce and N₂O harder to produce. It is possible to supply a sufficient amount of ammonia to the selective reduction type NO_(X) catalyst device and to sufficiently suppress the production of N₂O for which release into the atmosphere undesirable.

According to the exhaust purification system of an internal combustion engine as described in claim 2 according to the present invention, in the exhaust purification system of an internal combustion engine as set forth in claim 1, an interval from when ending the ammonia generation period to when next starting the ammonia generation period, for example, the time interval or running distance interval, when the temperature of the storage reduction type NO_(X) catalyst device is less than the set temperature is shorter than that when the temperature of the storage reduction type NO_(X) catalyst device is the set temperature or more such that the amount of NO_(X) which is held at the storage reduction type NO_(X) catalyst device for starting the ammonia generation period when the temperature of the storage reduction type NO_(X) catalyst device is less than the set temperature is smaller than that when the temperature of the storage reduction type NO_(X) catalyst device is the set temperature or more. Due to this simple control, even if the ammonia generation period is started when the storage reduction type NO_(X) catalyst device is a low temperature, it is possible to make ammonia easier to produce and N₂O harder to produce.

According to the exhaust purification system of an internal combustion engine as described in claim 3 according to the present invention, in the exhaust purification system of an internal combustion engine as set forth in claim 1, when the temperature of the storage reduction type NO_(X) catalyst device is another set temperature higher than the set temperature or more, instead of the ammonia generation period, the system makes the air-fuel ratio of the exhaust gas which flows into the selective reduction type NO_(X) catalyst device change from the lean air-fuel ratio to the stoichiometric air-fuel ratio or another rich air-fuel ratio close to the stoichiometric air-fuel ratio. Here, if starting the ammonia generation period when the temperature of the storage reduction type NO_(X) catalyst device is another set temperature higher than the set temperature or more and the reducing ability of the storage reduction type NO_(X) catalyst device is extremely high, a large amount of ammonia would be produced and a relatively large amount of ammonia would pass straight through the selective reduction type NO_(X) catalyst device. Due to this, at this time, instead of the ammonia generation period, the air-fuel ratio of the exhaust gas which flows into the selective reduction type NO_(X) catalyst device is made to change from the lean air-fuel ratio to the stoichiometric air-fuel ratio or another rich air-fuel ratio close to the stoichiometric air-fuel ratio so that the storage reduction type NO_(X) catalyst device purifies by reduction the NO_(X) which is disassociated from the storage reduction type NO_(X) catalyst device and the NO_(X) in the exhaust gas without almost any of the NO_(X) being made ammonia. In this way, in addition to the effects of the exhaust purification system of the internal combustion engine as set forth in claim 1, when the storage reduction type NO_(X) catalyst device is a high temperature and the reducing ability is extremely high, the ammonia generation period is not started and the storage reduction type NO_(X) catalyst device is made to purify NO_(X) by reduction so as to prevent a relatively large amount of ammonia from passing straight through the selective reduction type NO_(X) catalyst device.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view which shows an embodiment of an exhaust purification system of an internal combustion engine according to the present invention.

FIG. 2 is a flow chart for producing ammonia in a storage reduction type NO_(X) catalyst device, which is carried out in the exhaust purification system according to the present invention.

DESCRIPTION OF EMBODIMENTS

FIG. 1 is a schematic view which shows an exhaust purification system of an internal combustion engine according to the present invention. In the figure, 1 indicates an exhaust passage of a diesel engine or a direct-fuel-injection type spark ignition internal combustion engine carrying out lean combustion. The exhaust gas in the internal combustion engine carrying out lean combustion contains a relatively large amount of NO_(X), so a selective reduction type NO_(X) catalyst device 2 is arranged in the exhaust passage 1 for purifying the NO_(X). This selective reduction type NO_(X) catalyst device 2 has an ammonia holding ability by which it holds (or adsorbs) the ammonia when the concentration of the ammonia NH₃ in the exhaust gas is high and disassociates (or releases) the ammonia when the concentration of ammonia NH₃ in the exhaust gas becomes low and is, for example, formed as a zeolite-based denitration catalyst device comprised of a carrier on the surface of which copper zeolite, platinum-copper zeolite, or iron zeolite is carried or is formed including zeolite, silica, silica alumina, titania, or other solid acid and carrying iron Fe or copper Cu or other transition metal or platinum Pt or palladium Pd or other precious metal.

In such a selective reduction type NO_(X) catalyst device 2, the NO_(X) which is contained in the exhaust gas of the lean air-fuel ratio at the time of lean combustion is reduced by the ammonia NH₃ which is released from the selective reduction type NO_(X) catalyst device 2 (for example, 4NH₃+4NO+O₂→4N₂+6H₂O and 8NH₃+6NO₂→7N₂+12H₂O). In this way, if the selective reduction type NO_(X) catalyst device 2 is made to adsorb a sufficient amount of ammonia, it is possible to purify the NO_(X) in the exhaust gas well by reduction.

In the present exhaust purification system, to produce the ammonia to be supplied to the selective reduction type NO_(X) catalyst device 2, a storage reduction type NO_(X) catalyst device 3 is arranged at the upstream side of the selective reduction type NO_(X) catalyst device 2. The storage reduction type NO_(X) catalyst device 3 is formed by a carrier on which an NO_(X) holding agent and platinum Pt or another such precious metal catalyst are carried. The NO_(X) holding agent is at least one agent which is selected from potassium K, sodium Na, lithium Li, cesium Cs, or other such alkali metal, barium Ba, calcium Ca, or other such alkali earth, and lanthanum La, yttrium Y, or other such rare earth.

The storage reduction type NO_(X) catalyst device 3 holds the NO_(X) in the exhaust gas well, that is, absorbs it as nitrates well or adsorbs it well as NO₂ when the exhaust gas is a lean air-fuel ratio, that is, the concentration of oxygen in the exhaust gas is high. On the other hand, if making the air-fuel ratio of the exhaust gas the stoichiometric air-fuel ratio or the rich air-fuel ratio, that is, if making the concentration of oxygen in the exhaust gas low, the held NO_(X) is disassociated, that is, the absorbed NO_(X) is released and, further, the adsorbed NO₂ is disassociated. The disassociated NO_(X) is reduced by the reducing substances in the exhaust gas.

In the reduction of NO_(X), if the air-fuel ratio of the exhaust gas is the desired rich air-fuel ratio, when the temperature of the storage reduction type NO_(X) catalyst device 3 is high, NO_(X) is sufficiently reduced and mainly ammonia NH₃ and N₂ are produced (for example, 5H₂+2NO→2NH₃+2H₂O, 7H₂+2NO₂→2NH₃+4H₂O, 2CO+2N→N₂+2CO₂, 2H₂+2NO→2N₂+2H₂O, 4CO+2NO₂→N₂+4CO₂, 4H₂+2NO₂→N₂+4H₂O) . In this way, according to the storage reduction type NO_(X) catalyst device 3, not only part of the NO_(X) in the exhaust gas, but also part of the disassociated NO_(X) can be reduced to ammonia. A sufficient amount of ammonia NH₃ can be produced and supplied to the selective reduction type NO_(X) catalyst device 2.

However, if the storage reduction type NO_(X) catalyst device is a low temperature, the NO_(X) is reduced, due to insufficient reduction, not to N₂ and ammonia, but mainly dinitrogen monoxide N₂O (for example, 2NO+N₂→2N₂O and 2NO₂+3N₂→4N₂O) and N₂. Not only cannot a sufficient amount of ammonia be supplied to the selective reduction type NO_(X) catalyst device, but a relatively large amount of N₂O for which release into the atmosphere is undesirable ends up being produced.

To alleviate this problem, the exhaust purification system according to the present invention produces ammonia in the storage reduction type NO_(X) catalyst device 3 according to the flow chart which is shown in FIG. 2.

First, at step 101, it is judged if a current temperature T of the storage reduction type NO_(X) catalyst device 3 is less than a first set temperature Ti (for example, 300° C.). Here, the current temperature T of the storage reduction type NO_(X) catalyst device 3 may be measured by a temperature sensor, but may also be estimated. For example, the current engine operating state (engine speed, fuel injection amount, combustion air-fuel ratio, etc.) may be used as the basis to estimate the temperature T of the storage reduction type NO_(X) catalyst device 3. Further, the temperature of the exhaust gas (measured or estimated) which flows into the storage reduction type NO_(X) catalyst device 3 may be used as the basis to estimate the temperature T of the storage reduction type NO_(X) catalyst device 3.

When the judgment at step 101 is negative, at step 102, it is judged that the temperature (T) of the storage reduction type NO_(X) catalyst device 3 is a second set temperature (for example 400° C.) or more. When this judgment is negative, that is, the temperature (T) of the storage reduction type NO_(X) catalyst device 3 is the first set temperature (T1) or more and less than the second set temperature (T2), at step 103, it is judged if the amount (A) of NO_(X) which is held at the storage reduction type NO_(X) catalyst device 3 has reached a first set amount (A1).

Here, the amount (A) of the NO_(X) which is held at the storage reduction type NO_(X) catalyst device 3 can be estimated, for example, by setting in advance the amount of NO_(X) which is contained in exhaust gas per unit time for each engine operating state, assuming that a given set rate of it is held at the storage reduction type NO_(X) catalyst device 3 per unit time, and cumulatively adding the held amount per unit time.

When the judgment at step 103 is negative, the routine is ended as is. At this time, the NO_(X) which was not held at the storage reduction type NO_(X) catalyst device 3 is purified by reduction at the selective reduction type NO_(X) catalyst device 2 by using the disassociated ammonia. If the judgment at step 103 is positive, at step 104, to start the ammonia generation period at the storage reduction type NO_(X) catalyst device 3, the air-fuel ratio (AF) of the exhaust gas which flows into the storage reduction type NO_(X) catalyst device 3 is changed from the lean air-fuel ratio (AFL) at the time of lean combustion to the first rich air-fuel ratio (AFR1). For this reason, for example, it is possible to supply additional fuel to the exhaust passage 1 at the upstream side of the storage reduction type NO_(X) catalyst device 3 or supply additional fuel from a fuel injector in the exhaust stroke or expansion stroke into a cylinder.

Next, at step 105, it is judged if the elapsed time (t) from when the ammonia generation period was started has reached the first set time (t1). Up to when this judgment is positive, the air-fuel ratio (AF) of the exhaust gas which flows into the storage reduction type NO_(X) catalyst device 3 is made the first rich air-fuel ratio (AFR1). For example, the first set time (t1) is made the time until all of the first set amount A1 of NO_(X) which is held at the storage reduction type NO_(X) catalyst device 3 is disassociated by the exhaust gas of the first rich air-fuel ratio (AFR1).

When the judgment at step 105 is positive, that is, when the ammonia generation period reaches the first set time t1, at step 106, the air-fuel ratio (AF) of the exhaust gas which flows into the storage reduction type NO_(X) catalyst device 3 is changed to the lean air-fuel ratio (AFL) at the time of lean combustion. That is, the supply of additional fuel into the exhaust passage 1 or cylinder is stopped. In this way, the ammonia generation period is ended. From this time, cumulative addition of the NO_(X) amount which is held at the storage reduction type NO_(X) catalyst device 3 is started based on the current engine operating state. Further, the NO_(X) which was not held at the storage reduction type NO_(X) catalyst device 3 is purified by reduction using the disassociated ammonia at the selective reduction type NO_(X) catalyst device 2.

In this way, when the temperature (T) of the storage reduction type NO_(X) catalyst device 3 is relatively high and the storage reduction type NO_(X) catalyst device 3 has a sufficient reducing ability, in the ammonia generation period, as explained earlier, the NO_(X) in the exhaust gas and the NO_(X) which is disassociated from the storage reduction type NO_(X) catalyst device 3 are mainly reduced to ammonia and nitrogen, and a sufficient amount of ammonia NH₃ is produced and supplied to the selective reduction type NO_(X) catalyst device 2 to be held at the selective reduction type NO_(X) catalyst device 2.

However, when the temperature (T) of the storage reduction type NO_(X) catalyst device 3 is relatively low and the reducing ability of the storage reduction type NO_(X) catalyst device 3 falls, if the ammonia generation period is started in the same way as above, the NO_(X) in the exhaust gas and the NO_(X) which was disassociated from the storage reduction type NO_(X) catalyst device 3 end up being reduced to mainly nitrogen and dinitrogen monoxide.

In the present flow chart, when the judgment of step 101 is positive, at step 107, it is judged if the NO_(X) amount (A) which is being held at the storage reduction type NO_(X) catalyst device 3 has reached a second set amount (A2). The second set amount (A2) is an amount smaller than the first set amount (A1).

When the judgment at step 107 is negative, the routine is ended as it is. The NO_(X) which was not held at the storage reduction type NO_(X) catalyst device 3 is purified by reduction using the disassociated ammonia at the selective reduction type NO_(X) catalyst device 2. On the other hand, if the judgment of step 107 is positive, at step 108, to start the ammonia generation period at the storage reduction type NO_(X) catalyst device 3, in the same way as above, the air-fuel ratio (AF) of the exhaust gas which flows into the storage reduction type NO_(X) catalyst device 3 is changed from the lean air-fuel ratio (AFL) at the time of lean combustion to the first rich air-fuel ratio (AFR1).

Next, at step 109, it is judged if the elapsed time (t) from when the ammonia generation period was started has reached a second set time (t2). Until this judgment is positive, the air-fuel ratio (AF) of the exhaust gas which flows into the storage reduction type NO_(X) catalyst device 3 is made the first rich air-fuel ratio (AFR1). For example, the second set time (t2) is made the time until all of the second set amount (A2) of NO_(X) which is held at the storage reduction type NO_(X) catalyst device 3 is disassociated by the exhaust gas of the first rich air-fuel ratio (AFR1).

When the judgment at step 109 is positive, that is, when the ammonia generation period reaches the second set time (t2), at step 110, the air-fuel ratio (AF) of the exhaust gas which flows into the storage reduction type NO_(X) catalyst device 3 is changed to the lean air-fuel ratio (AFL) at the time of lean combustion. In this way, the ammonia generation period is ended. The cumulative addition of the NO_(X) amount which is held at the storage reduction type NO_(X) catalyst device 3 is started based on the current engine operating state from this time. Further, the NO_(X) which was not held at the storage reduction type NO_(X) catalyst device 3 is purified by reduction using the disassociated ammonia at the selective reduction type NO_(X) catalyst device 2.

In this way, when the temperature (T) of the storage reduction type NO_(X) catalyst device 3 is relatively low and the storage reduction type NO_(X) catalyst device 3 does not have a sufficient reducing ability, the ammonia generation period is started when the amount of NO_(X) which is held at the storage reduction type NO_(X) catalyst device 3 is small, the amount of NO_(X) which is disassociated from the storage reduction type NO_(X) catalyst device 3 in the ammonia generation period is made smaller, and the NO_(X) which is disassociated from the storage reduction type NO_(X) catalyst device 3 and the NO_(X) in the exhaust gas can be reduced to mainly ammonia and nitrogen.

Further, when the temperature (T) of the storage reduction type NO_(X) catalyst device 3 is extremely high and the reducing ability of the storage reduction type NO_(X) catalyst device 3 is extremely high, if starting the ammonia generation period such as at step 103, a large amount of ammonia would be produced and a relatively large amount of ammonia would end up passing straight through the selective reduction type NO_(X) catalyst device.

Due to this, when the judgment at step 102 is positive, at step 111, it is judged if the amount (A) of the NO_(X) which is held at the storage reduction type NO_(X) catalyst device 3 has reached the first set amount (A1). When the judgment at step 111 is negative, the routine is ended as it is When the judgment at step 111 is positive, at step 112, the air-fuel ratio (AF) of the exhaust gas which flows into the storage reduction type NO_(X) catalyst device 3 is changed from the lean air-fuel ratio (AFL) at the time of lean combustion to a second rich air-fuel ratio (AFR2). The second rich air-fuel ratio (AFR2) is a rich air-fuel ratio closer to the stoichiometric air-fuel ratio than the first rich air-fuel ratio (AFR1). Further, the air-fuel ratio (AF) of the exhaust gas which flows into the storage reduction type NO_(X) catalyst device 3 may also be made the stoichiometric air-fuel ratio instead of the second rich air-fuel ratio (AFR2).

In this way, if making the air-fuel ratio of the exhaust gas the stoichiometric air-fuel ratio or a rich air-fuel ratio (AFR2) near the stoichiometric air-fuel ratio, the reducing ability at the storage reduction type NO_(X) catalyst device 3 falls, the NO_(X) which is disassociated from the storage reduction type NO_(X) catalyst device 3 and the NO_(X) in the exhaust gas are purified by reduction mainly to nitrogen, and almost no ammonia is produced or the amount of production of ammonia can be sufficiently decreased to enable the selective reduction type NO_(X) catalyst device 2 to hold it.

By purifying NO_(X) by reduction in this way instead of starting the ammonia generation period, a relatively large amount of ammonia is prevented from passing straight through the selective reduction type NO_(X) catalyst device.

Next, at step 113, it is judged if the elapsed time (t) from the start of purification of NO_(X) by reduction to make the air-fuel ratio of the exhaust gas the second rich air-fuel ratio (AFR2) has reached a third set time (t3). Until this judgment is positive, the air-fuel ratio (AF) of the exhaust gas which flows into the storage reduction type NO_(X) catalyst device 3 is made the second rich air-fuel ratio (AFR2). For example, the third set time (t3) is made the time until all of the first set amount (A1) of NO_(X) which is held at the storage reduction type NO_(X) catalyst device 3 is disassociated by the exhaust gas of the second rich air-fuel ratio (AFR2).

When the judgment at step 113 is positive, at step 114, the air-fuel ratio (AF) of the exhaust gas which flows into the storage reduction type NO_(X) catalyst device 3 is changed to the lean air-fuel ratio (AFL) at the time of lean combustion. In this way, the purification of NO_(X) by reduction is ended. From this time, cumulative addition of the amount of NO_(X) which is held at the storage reduction type NO_(X) catalyst device 3 is started based on the current engine operating state.

When the temperature (T) of the storage reduction type NO_(X) catalyst device 3 is extremely high, the NO_(X) holding ability of the storage reduction type NO_(X) catalyst device 3 is also high and almost all of the NO_(X) in the exhaust gas is held at the storage reduction type NO_(X) catalyst device 3. In this way, at this time, almost all of the NO_(X) in the exhaust gas is purified by reduction as explained above after being held at the storage reduction type NO_(X) catalyst device 3. Therefore, the selective reduction type NO_(X) catalyst device 2 may not almost purify NO_(X) by reduction.

In the ammonia generation period (step 104 and 108) and the purification of NO_(X) by reduction (step 112) at the present flow chart, the air-fuel ratio of the exhaust gas may be continuously made the first rich air-fuel ratio (AFR1) or the second rich air-fuel ratio (AFR2), but it is also possible to make it so that the first rich air-fuel ratio (AFR1) or the second rich air-fuel ratio (AFR2) and the lean air-fuel ratio are repeated. In this case, the set times (t1), (t2), and (t3) until all of the NO_(X) which is held at the storage reduction type NO_(X) catalyst device 3 is disassociated are made longer compared to when continuously making the air-fuel ratio of the exhaust gas the first rich air-fuel ratio (AFR1) or the second rich air-fuel ratio (AFR2).

In the present flow chart, when the temperature (T) of the storage reduction type NO catalyst device 3 is the second set temperature T2 or more, instead of starting the ammonia generation period, the NO_(X) is purified by reduction (steps 111 to 114), but when the temperature (T) of the storage reduction type NO_(X) catalyst device 3 becomes the first set temperature T1 or more, it is also possible to not start the ammonia generation period, but purify the NO_(X) by reduction (steps 111 to 114).

Further, in the present flow chart, the amount (A) of the NO_(X) which is held at the storage reduction type NO_(X) catalyst device 3 is estimated so as to start the ammonia generation period or the purification of NO_(X) by reduction, but it is also possible to start the ammonia generation period or purification of NO_(X) by reduction every set time or set running distance. In this case, if the set time or the set running distance from the end of the ammonia generation period to the start of the next ammonia generation period when the temperature of the storage reduction type NO_(X) catalyst device is less than the first set temperature (T1) is made shorter than that when the temperature of the storage reduction type NO_(X) catalyst device is the first set temperature (T1) or more, it is possible to start the ammonia generation period when the amount of NO_(X) which is held at the storage reduction type NO_(X) catalyst device is small when the temperature of the NO_(X) catalyst device is less than the first set temperature as opposed to when the temperature of the NO_(X) catalyst device is the first set temperature or more.

LIST OF REFERENCE NUMERALS

-   1 exhaust passage -   2 selective reduction type NO_(X) catalyst device -   3 storage reduction type NO_(X) catalyst device 

1. An exhaust purification system of an internal combustion engine which is provided with a selective reduction type NO_(X) catalyst device which can hold ammonia and a storage reduction type NO_(X) catalyst device which is arranged at an upstream side of said selective reduction type NO_(X) catalyst device, which makes an air-fuel ratio of exhaust gas which flows into said storage reduction type NO_(X) catalyst device change from a lean air-fuel ratio to a rich air-fuel ratio for starting an ammonia generation period, and which makes an air-fuel ratio of exhaust gas which flows into said storage reduction type NO_(X) catalyst device change from a rich air-fuel ratio to a lean air-fuel ratio for ending said ammonia generation period, characterized in that an amount of NO_(X) which is held at said storage reduction type NO_(X) catalyst device for starting said ammonia generation period when a temperature of said storage reduction type NO_(X) catalyst device is less than a set temperature is smaller than that when a temperature of said storage reduction type NO_(X) catalyst device is said set temperature or more.
 2. An exhaust purification system of an internal combustion engine as set forth in claim 1, wherein an interval from when ending said ammonia generation period to when next starting said ammonia generation period when the temperature of said storage reduction type NO_(X) catalyst device is less than said set temperature is made shorter than that when the temperature of said storage reduction type NO_(X) catalyst device is said set temperature or more such that the amount of NO_(X) which is held at said storage reduction type NO_(X) catalyst device for starting said ammonia generation period when the temperature of said storage reduction type NO_(X) catalyst device is less than said set temperature is smaller than that when the temperature of said storage reduction type NO_(X) catalyst device is said set temperature or more.
 3. An exhaust purification system of an internal combustion engine as set forth in claim 1, wherein when the temperature of said storage reduction type NO_(X) catalyst device is another set temperature which is higher than said set temperature or more, instead of said ammonia generation period, making the air-fuel ratio of the exhaust gas which flows into said selective reduction type NO_(X) catalyst device change from a lean air-fuel ratio to the stoichiometric air-fuel ratio or another rich air-fuel ratio closer to the stoichiometric air-fuel ratio than said rich air-fuel ratio and making said storage reduction type NO_(X) catalyst device purify the NO_(X) which is disassociated from said storage reduction type NO_(X) catalyst device and the NO_(X) in the exhaust gas by reduction. 